1. Field of the Invention
The present invention relates to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that is capable of effectively performing an image density adjustment.
2. Discussion of the Background
In order to maintain high image quality using image forming apparatuses such as digital copying machines, printers, and facsimile machines which form an image on a photoconductive member with a laser beam, stabilizing a half-tone image density has been considered a major factor. One example of an image forming apparatus which attempts to stabilize the half-tone image density is described in Japanese Laid-Open Patent Publication No. 6-208271. This image forming apparatus has a configuration whereby several calculations are performed. A control voltage which controls a charging mechanism for charging a photoconductive member at a desired dark voltage is calculated. An amount of a laser beam for obtaining a desired half-tone image voltage is calculated based on a voltage of a latent image patch which is formed on the photoconductive member charged at the desired dark voltage and a light amount for producing the latent image patch. A development bias voltage is calculated by subtracting the desired dark voltage by a predetermined voltage. Based on these calculations, the control voltage, the amount of the laser beam, and the development bias voltage are updated. However, this technique requires voltage detection. Since voltage detection equipment normally is an expensive tool, the manufacturing cost of the image forming apparatus is increased.
Another example of an image forming apparatus which attempts to stabilize the halftone image density is described in Japanese Laid-Open Patent Publication No. 8-265570. This image forming apparatus includes a toner pattern generating mechanism for generating a toner image density detection pattern on an image bearing member, an image density detection mechanism for detecting an image density of a toner image, a controlling mechanism for controlling the image density in accordance with the detection result, and a data table for representing a gray-scale based on a relationship between the detection result and the image density. However, this technique requires a relatively long time period to perform the detection of a half-tone image since it generates a number of half-tone image patterns during the time of such detection. Moreover, since this technique conducts the adjustment of the half-tone image density during a time period of a power wide modulation of a laser diode, the accuracy with respect to a width of the adjustment is relatively rough.
Another example of an image forming apparatus which attempts to control an amount of toner deposition in a range of images from a solid black image to a half-tone image is described in Japanese Laid-Open Patent Publication No. 8-297834. This image forming apparatus includes a test patch generating mechanism for generating test patches for solid black and half-tone images using toner on an image carrying member and an image density detection mechanism for detecting image density of the test patches. With such a configuration, the image forming apparatus controls a density of a solid black image by controlling a rotation speed of the image carrying member based on the detection result with respect to the solid black image. Then, detection of image density with respect to the test patch of the half-tone image is performed and, based on the detection result, the density of the half-tone image is controlled by control of a laser power. However, controlling the rotation speed of the image bearing member requires an expensive mechanism which increases the manufacturing cost of the image forming apparatus. Moreover, this configuration may cause an overlapping of a background image or an erroneous white mark in a rear edge of a half-tone image by changing the rotation speed of the image carrying member, particularly, in a case that the image forming apparatus uses a development agent of two components and a magnetic brush. In addition, the above-mentioned three image forming apparatuses commonly use a scorotron charger, that is, a corona charger having a screen electrode and which is prone to produce ozone.
A contact type charger such as a charging roller is known to produce less ozone but also to cause variations of charge voltage to a relatively great extent. The variations of the charge voltage can be reduced with a technique which corrects a charge output using a sensor for detecting a reflection density. However, this technique also has a drawback in that the difference between the charge voltages before and after the adjustment adversely affects a voltage of the half-tone image and, as a result, the image density of the half-tone image is varied.
Accordingly, an object of the invention is to provide a novel image forming apparatus. The novel image forming apparatus includes a photoconductive member, a charging mechanism, an optical writing mechanism, a development mechanism, a sensing mechanism, and a controlling mechanism. The charging mechanism is configured to charge a surface of the photoconductive member in order that the surface is charged at a charge voltage. The optical writing mechanism is configured to write latent images including a first latent image and a second latent image on the surface of the photoconductive member charged at the charge voltage. The development mechanism is configured to develop the first latent image into a first toner pattern at a solid toner density and the second latent image into a second toner pattern at a half-tone tone density. The sensing mechanism is configured to detect reflection densities of the first and second toner patterns and to generate output signals representing detection results detected by the sensing mechanism. The controlling mechanism is configured to adjust values of the solid toner density, the charge voltage, and the half-tone toner density based on the output signals generated by the sensing mechanism. In the above-mentioned novel image forming apparatus, the controlling mechanism adjusts the value of the charge voltage by changing a voltage to be applied to the charging mechanism at intervals of a predetermined time period and adjusts the value of the half-tone toner density by controlling the optical writing mechanism to change a light amount.
The charging mechanism may include a charging roller configured to apply a charge to the photoconductive member in contact thereto.
The optical writing mechanism may write a third latent image on the surface of the photoconductive member charged at the charge voltage and the development mechanism may develop the third latent image into a third toner pattern at a solid toner density so that the first toner pattern is used to adjust the value of the solid toner density and the third toner pattern is used to adjust a value of the charge voltage.
The controlling mechanism is configured to adjust the value of the half-tone toner density based on the output signals generated by the sensing mechanism upon completing the adjustment of the solid toner density and the charge voltage based on the output signals generated by the sensing mechanism after the completion of a job.
The controlling mechanism is configured to adjust the value of the half-tone toner density by controlling the optical writing mechanism to change the light amount at multiple levels so that the apparatus forms a multi-level half-tone image.
The image forming apparatus is configured to form the multi-level half-tone image using an error diffusion.
The present invention further provides a novel method for image density adjustment. The novel method of image density adjustment includes the steps of charging, optically writing, developing, detecting, generating, and adjusting. The charging step charges a surface of a photoconductive member so that the surface is charged at a charge voltage. The optically writing step writes latent images including a first latent image and a second latent image on the surface of the photoconductive member charged at the charge voltage. The developing step develops the first latent image into a first toner pattern at a solid toner density and the second latent image into a second toner pattern at a half-tone tone density. The detecting step detects reflection densities of the first and second toner patterns. The generating step generates output signals representing results of the detecting step. The adjusting step adjusts values of the solid toner density, the charge voltage, and the half-tone toner density based on the output signals generated by the generating step. In the above described novel method, the adjusting step adjusts the value of the charge voltage by changing a voltage to be applied in the charging step at intervals of a predetermined time period and the value of the half-tone toner density by controlling the optically writing step to change a light amount.
The charging step may use a charging roller configured to apply a charge to the photoconductive member in contact thereto.
The optically writing step may write a third latent image on the surface of the photoconductive member charged at the charge voltage and the development step may develop the third latent image into a third toner pattern at a solid toner density so that the first toner pattern is used to adjust the value of the solid toner density and the third toner pattern is used to adjust value of the charge voltage.
The adjusting step may adjust the value of the half-tone toner density based on the output signals generated in the generating step upon completing the adjustment of the solid toner density and the charge voltage based on the output signals generated in the generating step after the completion of a job.
The adjusting step may adjust the value of the half-tone toner density by controlling the optically writing step to change the light amount at multiple levels so that the method is applied to a multi-level half-tone image.
The method for image density adjustment may be applied to the multi-level half-tone image using an error diffusion.